Method and apparatus for pulse generation and adaptive pulse generation for optical communications

ABSTRACT

One embodiment of the invention relates to producing optical pulses for use on a transmission link. A light source is configured to produce an optical signal. A pulse generator is coupled to the light source. The pulse generator is configured to receive, for a first channel, the optical signal and a clock signal. The pulse generator is also configured to modify the optical signal based on the clock signal to produce an optical pulse having a predetermined pulse shape. The clock signal is associated with the predetermined pulse shape. The predetermined pulse shape being based on a transmission characteristic of the transmission link

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to co-pending U.S. patentapplication Ser. No. 10/084,057, entitled “Method and System formitigating nonlinear transmission impairments in fiber-opticcommunications systems,” filed on Feb. 28, 2002, which claims priorityto No. 60/352,991, entitled “Optical communication system and method,”filed on Feb. 1, 2002; both the entirety of which are incorporatedherein by reference. This application is also continuation-in-part ofthe patent application “Light source for generating output signal havingevenly spaced apart frequencies,” filed on Jun. 18, 2002, the entiretyof which is incorporated herein by reference.

BACKGROUND

[0002] The invention relates generally to optical communications such asa system and method for optical communications with data rates of atleast 10 Gbit/s.

[0003] Many of optical communication systems with data rates of at least10 Gbit/s are based on a return-to-zero (RZ) format for datatransmission. Such known optical communication systems typically relateto generating an optimal pulse shape or to evaluating the pulse shapeonce generated.

[0004] For example, adaptive pulse shaping in free space and in the 850nm spectral range had been disclosed in D. Yelin et al., “Adaptivefemtosecond pulse compression,” Opt. Letters, 1997, v.22, #23, pp.1793-1795. The input pulse arrives from a comb generator (in this case,a mode-locked laser), and is spatially separated into its spectralcomponent by a grating. A lens maps each wavelength group onto aseparate pixel of a computer-controlled phase modulator. A second lensand grating recombine the components back together to form a controlledoutput pulse. A doubler crystal samples the output pulse. This crystalproduces pulses that are higher the shorter the pulse is. By using anappropriate algorithm, a computer can adapt the phases of the differentspectral components of the input pulses so that for each type of inputpulse, the shortest pulse possible should be produced by the device. Theshortest pulse is considered as the most optimal for the communicationsystem.

[0005] An alternative but analogous method was disclosed in K. Kitayamaet al. “Optical pulse train synthesis of arbitrary waveform usingweight/phase-programmable 32-tapped delay line waveguide filter,”Proceedings of OFC-2001, paper WY3-1. Similar to the Yelin system, thepulse source in Kitayama is a comb generator, but the pulse shaping isdone through a parallel series of delay lines and attenuators.

[0006] These known devices, however, suffer several shortcomings. Forexample, these known devices are quite complex from the technical pointof view. In addition, because these known devices typically relate togenerating an optimal pulse shape or to evaluating the pulse shape oncegenerated, such devices are bulky, expensive and not appropriate for usein commercial systems.

[0007] Moreover, in real optical communication system, eitherterrestrial or undersea, the fiber conditions and multiple componentoperations change in time. Therefore, the optimal pulse shape isdifferent for the every particular time interval. The best performanceof the pulse generator or pulse shaper should include a closed loop tocorrect adaptively the changing conditions. An adaptive approach for thepulse shaping in fiber communication has been developed by a number ofresearch groups (see, for example, F. G. Omenetto, M. D. Moores, B. P.Luce, D. H. Reitze and A. J. Taylor “Femtosecond pulse delivery throughsingle-mode optical fiber with adaptive pulse shaping,” ProceedingsCLEO'2001, pp. 234-235). Indeed, such an approach can provide amechanism to overcome multiple limitations associated with nonlineareffects and provides an opportunity to synthesize pulses that areself-correcting for higher order nonlinear effects when being launchedin the fiber.

[0008] As discussed below, in the present invention, a device isdescribed that can be implemented in real RZ communication systems andcan provide a number of advantages from the point of view of chromaticdispersion reduction and nonlinear effects mitigation. This results inan improvement of the communication link figure of merit:cost/(capacity*distance). The described device provides new technicalsolutions in the pulse formation and in the pulse shape evaluationtogether with adaptive shaping in time.

SUMMARY OF THE INVENTION

[0009] One embodiment of the invention relates to producing opticalpulses for use on a transmission link. A light source is configured toproduce an optical signal. A pulse generator is coupled to the lightsource. The pulse generator is configured to receive, for a firstchannel, the optical signal and a clock signal. The pulse generator isalso configured to modify the optical signal based on the signal toproduce an optical pulse having a predetermined pulse shape. The signalis associated with the predetermined pulse shape. The predeterminedpulse shape being based on a transmission characteristic of thetransmission link.

[0010] Another embodiment of the invention relates to the generation ofpulses having a pre-determined shape using amplitude or phase modulationby generateingat least two signals at strong side harmonic frequenciesand combining them to create the pulse.

[0011] Another embodiment of the invention relates to the method anddevice for generating pulses having a pre-determined shape and pre-chirpusing a combination of amplitude or phase modulators and a slow phaseshifter. The slow phase shifter produces a chirp by introducing arelative phase shift between the carrier and the side harmonics.

[0012] Another embodiment of the invention relates to the method anddevice for generating pulses having a pre-determined pulse shape basedon comb-generator that produces light with a set of frequencies evenlyspaced apart.

[0013] Another embodiment of the invention relates to measuring anoptical pulse shape after being transmitted on a communication link. Aphotodiode is configured to receive an optical pulse having a firstspectral component, a second spectral component and a third spectralcomponent. The second spectral component and the third spectralcomponent are based on a clock frequency. The photodiode is configuredto send a first signal having an amplitude and a spectral component withthe clock frequency. A filter is coupled to the photodiode. The filterhas a spectral response associated with the clock frequency. A detectoris coupled to the filter. The detector is configured to send an errorsignal based on the amplitude of the first signal.

[0014] Another embodiment relating to measuring an optical pulse shapeincludes a dispersion device and a balanced detector. The dispersiondevice is configured to receive a first portion of an optical signal ona first optical path and a second portion of the optical signal on asecond optical path. The dispersion device is further configured tointroduce a first dispersion into the first portion of the opticalsignal and a second dispersion into the second optical signal. The firstdispersion has its own amplitude and sign. The second dispersion has itsown amplitude and sign. The amplitude of the first dispersion issubstantially equal to the amplitude of the second dispersion. The signof the first dispersion is opposite of the sign of the seconddispersion. The balanced detector coupled to the dispersion device.

[0015] Another embodiment relating to measuring an optical pulse shapeis a method based on autocorrelation. The device includes, for example,an optical hybrid, delay line and balanced detectors. In thisembodiment, as described below, the width of the incoming optical pulseis measured not in one short measurement but through a series ofmeasurements over multiple pulses.

[0016] Another embodiment of the invention relates to adaptive pulseshaping. An optical signal received from a transmission link ismeasured. The optical signal includes a set of optical pulses having anestimated pulse width. A first dispersion is introduced into a firstportion of an optical signal on a first optical path. A seconddispersion is introduced into a second portion of the optical signal ona second path. The first dispersion has its own amplitude and sign. Thesecond dispersion has its own amplitude and sign. The amplitude of thefirst dispersion is substantially equal to the amplitude of the seconddispersion. The sign of the first dispersion is opposite of the sign ofthe second dispersion. The first portion of the optical signal isdetected after the introducing of the first dispersion, and the secondportion of the optical signal is detected after the introducing of thesecond dispersion to produce a balanced-detected signal.

[0017] Another embodiment of the invention relates to the softwaredithering method for adaptive pulse shaping.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows a system block diagram of a communication system,according to an embodiment of the invention.

[0019]FIG. 2 shows a system block diagram of a pulse generator using aMach-Zehnder (MZ) modulator, according to an embodiment of theinvention.

[0020]FIG. 3 shows Q-factor as a function of the pulse shape parameterα₂ for two different transmission distances of 2000 and 1500 km.

[0021]FIGS. 4 and 5 show the normalized amplitude versus time ofdata-modulated signals corresponding those referred to FIG. 3 for the1500 km and 2000 km communication systems, respectively.

[0022]FIG. 6a shows the normalized intensity as a function of time foran example of quasi-RZ pulses having a linear chirp.

[0023]FIG. 6b shows the frequency versus time for the quasi-RZ pulsesshown in FIG. 6b.

[0024]FIG. 7 shows a pulse generator, according to an embodiment of theinvention.

[0025]FIG. 8 shows a pulse generator, according to another embodiment ofthe invention.

[0026]FIG. 9 shows a pulse generator, according to yet anotherembodiment of the invention.

[0027]FIG. 10 shows a comb-generator light source and a pulse generator,according to another embodiment of the invention.

[0028]FIG. 11 shows a pulse shape detector according to an embodiment ofthe invention.

[0029]FIG. 12 show the time response (in ps) of the pulse shape detectorshown in FIG. 11 for a modulation frequency F=12.5 GHz.

[0030]FIG. 13 shows a pulse shape detector based on autocorrelation,according to another embodiment of the invention.

[0031]FIG. 14 shows a pulse shape detector, according to anotherembodiment of the invention.

[0032]FIG. 15 shows a pulse shape detector with single chirped fiberBragg grating, according to an embodiment of the invention.

[0033]FIG. 16 shows a pulse shape detector with single chirped fiberBragg grating, according to another embodiment of the invention.

[0034]FIGS. 17a through 17 d shows an example of a series of pulsetrains as an original distorted pulse shape is adaptively corrected,resulting from the method described in connection with FIG. 18.

[0035]FIG. 18 shows a flowchart of a method for adaptive shapecorrection, according to an embodiment of the invention.

[0036]FIG. 19 shows the test setup for measuring pulse propagation in alink with significant dispersion, according to an embodiment of theinvention.

[0037]FIGS. 20a and 20 b show an example of the output pulse waveform onan oscilloscope based on the test setup of FIG. 22.

[0038]FIGS. 21a and 21 b show an example of the output pulse spectrum onan OSA corresponding to the output pulse waveform shown in FIGS. 20a and20 b.

[0039]FIG. 22 shows a test setup for testing the method for adaptivepulse shape correction described in connection with FIG. 17.

DETAILED DESCRIPTION

[0040]FIG. 1 shows a system block diagram of a communication system,according to an embodiment of the invention. As shown in FIG. 1, thecommunication system includes a light source 100, pulse generator 110,communication link 10, pulse shape detector 20, computer 130, andadaptive phase and attenuation control 120. The light source 100, pulsegenerator 110, adaptive phase and attention control 120 and computer 130are typically located within a transmitter system. The pulse shapedetector 20 is typically located within a receiver system. Thecommunication link 10 connects the transmitter system and the receiversystem.

[0041]FIG. 2 shows a system block diagram of a pulse generator using aMach-Zehnder (MZ) modulator, according to an embodiment of theinvention. As shown in FIG. 2, pulse generator 200 includes MZ modulator210, which can be for example an MZ LiNbO₃ modulator. MZ modulator 210receives continuous-wave (CW) light 201 and a clock signal 202 thatincludes a swing voltage and a voltage bias, and produces output opticalpulses 203. The pulse generator 110 shown in FIG. 1 can be embodied bypulse generator 210 of FIG. 2. CW light 201 can be provided to MZmodulator 210, for example, by light source 100 of FIG. 1. The clocksignal 202 can be provided to MZ modulator 210, for example, by adaptivephase and attenuation control 120.

[0042] In this embodiment, the temporal form of the pulse shape ofoutput optical pulses 203 can be given by:

I _(out)=sin[α₁+α₂ cos(2πt/T+π)],  (1)

[0043] where T is a bit period, t is time, α₁ and α₂ are variableparameters. When such a pulse shape is formed, for example, by passingCW light 201 through MZ modulator 210, α₁ is the swing voltage and α₂ isthe voltage bias.

[0044] The appropriate selection of values for parameters α₁ and α₂provides a pulse shape that is substantially optimal for particularfiber plant such as communication link 10 shown in FIG. 1. The term“optimal pulse shape” means a pulse shape that has reduced non-linearpenalties, reduced inter-channel interference and reduced chromaticdispersion distortions of the signal as compared to other possible pulseshapes. As a result of using substantially optimal shape pulses in acommunication system, the signal/noise ratio of the received signal issubstantially maximized. The value of the signal/noise ratio in afiber-optic communication system is usually described by the termQ-factor. Bit-error-rate (BER) is related to Q-factor asBER=exp(−Q²/2)/(Q(2π)^(1/2)) in the case of single detector. Forexample, a Q-factor value of 6 corresponds to a bit-error rate (BER)value of 10⁻⁹.

[0045] A given value of the Q-factor depends, at least in part, on theselection of α₁ and α₂ provided to the MZ modulator 210. FIG. 3 showstwo examples of a particular solution of the Q-factor as a function ofα₂, the voltage bias, according to the embodiment of the MZ modulatorshown in FIG. 2. More specifically, FIG. 3 shows the values of theQ-factor as a function of α₂, the voltage bias, for two long-haul (2000km and 1500 km) optical communication systems using binary phase shiftkeying (BPSK) data modulation. As FIG. 3 illustrates, the value of theQ-factor can be optimized by selecting, among other things, anappropriate value of the voltage bias of the clock signal provided to MZmodulator 210 by adaptive phase and attention control 130. Although notshown in FIG. 3, the selection of the swing voltage also affects thevalue of the Q-factor.

[0046]FIGS. 4 and 5 show the normalized amplitude versus time ofdata-modulated signals corresponding those referred to FIG. 3 for the1500 kin and 2000 km communication systems, respectively. As FIGS. 4 and5 show, the optimal pulse shape in this case is quasi-RZ signal.Simulations conducted by the inventors show that about the same valuesof α₁ and α₂ parameters will be appropriate for on-off-keying (OOK) datamodulation format in long haul optical fiber communication. In this caseof the OOK data modulation format, a quasi-RZ signal will also be asubstantially optimal pulse shape. A pulse temporal form based on othersmooth functions such as a Gaussian function and/or polynomials type offunctions may also be used. A higher order Gaussian, such as superGaussian, may also be used.

[0047] Other embodiments of the pulse generator are based on theformation of several harmonics of incoming CW light and the appropriatechoice of the relative phase and amplitude of these spectral components.Although some of the embodiments described herein use three spectralcomponents, a larger number of spectral components is also possible.Simulations conducted by the inventors show that the optimal pulseshapes for dense wavelength-division multiplexing (WDM) communicationsystems with phase modulation of data, quasi-RZ pulse shapes like thoseshown in FIGS. 4 and 5 for particular distances, can be formed by 5spectral harmonics with the accuracy of about 2%, and by 3 spectralcomponents with the accuracy of about 5%. These accuracies can beobtained by the appropriate selection of spectral components' amplitudesand relative phases. Alternatively, a variety of other pulse shapesbased on smooth functions can be formed.

[0048] In another embodiment of the invention, the pulses formed bysynthesis of several harmonics are pre-chirped. Pulse pre-chirping iswidely used in fiber communications to compensate for chromaticdispersion of communication line (i.e., chromatic dispersion that occurswithin the transmission fiber during propagation of light signals). Infiber communication links with compensated chromatic dispersion, someresidual dispersion that varies due to environmental changes exists. Thepulse pre-chirping can compensate for this residual dispersion. FIG. 6ashows the normalized intensity as a function of time for an example ofquasi-RZ pulses having a linear chirp. FIG. 6b shows the frequencyversus time for the quasi-RZ pulses shown in FIG. 6a. Three spectralharmonics with appropriate choice of relative amplitudes and phases wereused to obtain this pulse shape and pulse pre-chirp.

[0049] In other embodiments of the invention, other types of devices andmethods can be used to achieve a substantially optimal pulse shape andpulse pre-chirp. Unlike many known pre-chirping schemes where the pulseusually requires an additional fast phase modulator, certain embodimentsof the invention described below do not require an additional fast phasemodulator. For example, FIG. 7 shows a pulse generator, according to anembodiment of the invention. Pulse generator 700 includes an inputmodulator 710, an asymmetric MZ interferometer (AMZI) 720, phase shifter730 and an AMZI 740. Input modulator 710 can be, for example, a phase oramplitude modulator. Although not shown in FIG. 7, pulse generator 700can include an amplifier to drive the phase shifter 730.

[0050] CW light 701 of frequency f formed by, for example, a distributedfeedback laser (for example, JDS Uniphase, model CQF935/508) (not shownin FIG. 7) is provided to input modulator 710. The input modulator 710also receives a clock signal 703 with a bit rate F. The modulator 710can be, for example, a JDS Uniphase LiNbO₃ MZ modulator model 10020427.The output signal from the modulator 710 includes the carrier and twostrong side spectral components: f−F, f, f+F. The term “strong sidespectral components” refers to spectral components f−F and f+F, whichhave non-trivial amplitudes (i.e., amplitudes above the noise floor).

[0051] The signal output from the modulator 710 is provided to AMZI 720,which acts as a demultiplexer and has, for example, a free spectralrange (FSR) equal to 2F. AMZI 720 can be, for example, modelM0013NPMFP-DPXA by NTT Electronics. AMZI 720 performs signaldemultiplexing and divides the received light into different waveguides.Light having the spectral component with the frequency f is coupled intowaveguide 732, and the light having the spectral components withfrequencies f+F and f−F is coupled into waveguide 734.

[0052] Phase shifter 730 is coupled to waveguide 732 and receives aphase adjustment signal 705. Phase shifter 730 (for example, produced byJDS Uniphase LiNbO3 MZ modulator model #10024520) introduces chirp byphase modulating the light having the spectral component with frequencyf. Phase shifter 730 phase modulates the light propagating in waveguide732 based on the phase adjustment signal 705. In this manner, the phaseshifter 730 can introduce a desired chirp into the light having thespectral component with frequency f.

[0053] AMZI 740, having corresponding characteristics as AMZI 720,combines together the light. More specifically, AMZI 740 combines thelight having frequency f, with the light having frequencies f+F and f−Fto form output optical pulses 709 having a given pulse shape. In otherwords, similar to AMZI 720, which acts as a demultiplexer to separatethe light having different spectral components into differentwaveguides, AMZI 740 acts as a multiplexer to recombine the light havingdifferent spectral components from these different waveguides to produceoutput optical pulses 709.

[0054]FIG. 8 shows a pulse generator, according to another embodiment ofthe invention. Pulse generator 800 includes modulator 810, AMZI 820,phase shifter 830 and balanced loop mirror 840. Although not shown inFIG. 8, pulse generator 800 can include an amplifier to drive the phaseshifter 830.

[0055] Modulator 810 receives CW light 801, for example, from lightsource 100 and receives a modulation signal 803 having a bit rate F.Modulator 810 produces light having three spectral components: f, f+F,and f−F. The light is then provided to AMZI 820 having, for example, aFSR of 2F. Carrier light having the spectral component with thefrequency f is coupled into waveguide 832, and the light having thespectral components with frequencies f+F and f−F is coupled intowaveguide 834. Phase shifter 830 phase modulates the light propagatingin waveguide 832 based on the phase adjustment signal 805. In thismanner, the phase shifter 830 can introduce a desired chirp into thelight having the spectral component with frequency f.

[0056] After being modulated by phase shifter 830, the light fromwaveguide 832 and the light from waveguide 834 are reflected by balancedloop mirror 840. Thus, pulse generator 800 acts in a bi-directionalmanner. Instead of using a second AMZI (analogous to AMZI 740 shown inFIG. 7), AMZI 820 is used twice: once to split the light into waveguides832 and 834, and once again to combine light returning from waveguide832 and 834. The value of the phase modulation indicated by phaseadjustment signal 805 and applied to the phase shifter 830 should behalf that of phase adjustment signal 705 for the phase shifter 730 asshown in FIG. 7. This is due to the fact that the light passes throughphase shifter 830 twice, once in each direction.

[0057]FIG. 9 shows a pulse generator, according to yet anotherembodiment of the invention. Pulse generator 900 includes modulator 910,AMZI 920, phase shifter 930 and waveguides 932 and 934. Waveguides 932and 934 each have an end with a high reflection coating. Pulse generator900 is similar to pulse generator 800 of FIG. 8 where the balanced loopmirror 840 is replaced by the two high reflection coatings on the endsof waveguides 932 and 934. Although not shown in FIG. 9, pulse generator900 can include an amplifier to drive the phase shifter 930.

[0058] For the embodiments shown in FIGS. 7-9, output optical pulseshaving a desired pulse shape can be produced without the use ofphase-locked lasers. The shape of a generated pulse is controlled by thedrive on the phase modulator (e.g., modulator 710, 810 and 910) whilethe chirp is controlled by the phase shifter (e.g., phase shifter 730,830 and 930). Experiments conducted by the inventors have achieved achirp of −0.2 GHz/ps. In these experiments, the driving voltage of thephase modulator (e.g., modulator 710, 810 and 910) was V_(π)/3 and thephase on the phase shifter (e.g., phase shifter 730, 830 and 930) wasapproximately 0.77π.

[0059]FIG. 10 shows a comb-generator light source and a pulse generator,according to another embodiment of the invention. The light source 1090includes comb generator 1095. The pulse generator 1000 includesdemultiplexer 1010, phase shifter 1020 and multiplexer 1030. Althoughonly a single phase shifter 1020 and multiplexer 1030 are shown in FIG.10, this shown phase shifter 1020 and multiplexer 1030 arc associatedwith one communication channel. Thus, addition sets of phase shiftersand multiplexers (not shown) can be associated with additionalcommunication channels and included within pulse generator 1000.

[0060] In this embodiment, the light source 1090 can include a combgenerator 1095 producing an output with, for example, evenly spacedapart frequencies as described, for example, in the U.S. patentapplication entitled “Light Source for generating output signal havingequally spaced apart frequencies” filed on Jun. 18, 2002. Alternatively,other configurations of a comb generator can be used. Three, five ormore spectral components of the output of the comb generator 1095 can befiltered from the remaining spectral components to create the pulses ofone communication channel. Another set of spectral components can beused to form the pulses of another communication channel, and so on.Similar to the above-described embodiments of FIGS. 7 through 9, theembodiment shown in FIG. 10 using a comb generator can create theoptical pulses with a substantially optimal shape and pre-chirp.

[0061] More specifically, light output from comb generator 1095 isdemultiplexed by demultiplexer 1010. Three (or more) spectral componentsare used to create one communication channel. Phase shifters 1020 createthe desired pulse shape for each communication channel. The parametersof these phase shifters 1020 can be controlled adaptively to follow thechanges in the dispersion map and non-linear properties of thecommunication link (i.e., the optical transmission fiber).

[0062] In an alternative embodiment of a pulse generator, a portion ofthe pulse generator of FIG. 10 can be combined with a portion of thepulse generator of FIG. 7, 8 or 9. More specifically, a comb-generatorlight source can be coupled to a demultiplexer having an output channelbandwidth substantially equal to three (or more) spectral component ofthe comb-generator light source. In other words, each optical signaloutput from the demultiplexer has light with at least three spectralcomponents associated with a particular information channel. Eachoptical signal output from the demultiplexer can be received by thefirst AMZI shown in FIGS. 7, 8 or 9. This first AMZI can then isolatethe light having one spectral component (e.g., light having thefrequency f). A slow-phase modulator can then modulate the light. Thelight having the one spectral component and light having the remainingspectral components can then be combined to create the desired pulseshape in the respective communication channel.

[0063] The practical solutions for the pulse shape formation are notlimited to these described embodiments. Many other embodiments arepossible where the spectral components of the light are separated sothat the light having at least one spectral component can be modifiedthereby creating a desired pulse shape.

[0064] The pulse generators described above can be used in conjunctionwith a pulse shape detector located at the receiver side of thecommunication system (e.g., pulse shape detector 20 as shown in FIG. 1).To close the loop and have the subsequently transmitted pulse shape beadapted based on the dispersion of the link (e.g., communication link10), an error signal can be produced at the transmitter. This errorsignal should be a measure of the deviation from the nominal pulse shapedue to the transmission of a pulse through the communication link. Thiscan be accomplished in several ways such as, for example, measuring thepulse width using a standard method like oscilloscope or anautocorelator, and then modifying the pulse shape based on thedifference between the desired value and the actual value as the errorsignal.

[0065] The difference between the desired pulse shape and the actuallyreceived pulse shape can be determined in a number of ways. For example,one embodiment described below uses an autocorrelator approach.Alternatively, the physical properties of the received pulse that are astrong function of the pulse width, such as the pulse's second opticalharmonic or third optical harmonic, can be used. Regarding the secondoptical harmonic, see, for example, D. Yelin, D. Meshulach, and Y.Silberberg, “Adaptive femtosecond pulse compression”, Opt. Letters,1997, v.22, #23, pp. 1793-1795. Regarding the third optical harmonic,see, for example, D. Meshulach, Y. Barad, and Y. Silberberg “Measurementof ultrashort optical pulses by third-harmonic generation”, J. Opt. Soc.Am. B, vol.14, #4, pp.2122-2125. The magnitudes of these harmonics canbe used as the error signal. Yet another way is to use a system monitor,like BER data or eye diagrams to produce the error signal.

[0066]FIG. 11 shows a pulse shape detector according to an embodiment ofthe invention. In this embodiment, the magnitude of the detectedmicrowave signal is used as a characteristic of the pulse width. Morespecifically, a signal having a clock frequency F (initially used at thepulse generator to modulate the light having a frequency f) is isolatedand its magnitude is measured. This measured magnitude of the signal iscorrelated with the pulse width of the received optical signals.

[0067] The incoming optical signal 1101 includes at least three spectralcomponents f₁−F, f₁, f₁+F and is detected by PIN photodiode 1110. Theelectrical signal 1103 output from PIN photodiode 1110 includes a signalterm that corresponds to the intensity of the optical signal 1101 atfrequency F. Bandpass filter 1120 separates the spectral component atfrequency F of signal 1103 from the remaining spectral components ofsignal 1103 to produce signal 1105. Bandpass filter 1120 can be tuned,for example, to frequency F. The intensity of signal 1105 is detected byRF detector 1130 to produce signal 1107.

[0068]FIG. 12 show the time response (in ps) of the pulse shape detectorshown in FIG. 11 for a clock frequency F=12.5 GHz. Under realoperational conditions, only the right side of the curve in FIG. 12 isrelevant because optical pulses shorter than 25 ps are not typical inoptical communication systems. Consequently, a unique voltage can beassigned to each pulse width measured for received optical pulses. Thus,the time response shown in FIG. 12 can be used to define a desired errorsignal. Note that a modulation frequency of F=25 GHz corresponds to theminimal measurable pulse width equal to 12.5 ps.

[0069] For embodiments where the incoming optical pulse is phasemodulated with data, the spectral content of the optical pulse becomesmore complicated and can obstruct the proper operation of methods thatdepend on measuring the pulse width. Most communication systems,however, are designed so that un-modulated training pulses periodicallyoccur. For example, in a 12.5 GHz pulse train, 30 pulses for every 1000pulses can be un-modulated training pulses so that only fewpulses-per-minute (ppm) will be dedicated to training pulses rather thanpulses carrying data. This few number of un-modulated training pulses,however, can be sufficient to provide appropriate adaptive dispersioncontrol when in the right pulse rate and with an effectivesignal-to-noise ratio.

[0070]FIG. 13 shows a pulse shape detector based on autocorrelation,according to another embodiment of the invention. In this embodiment, asdescribed below, the width of the incoming optical pulse is measured notin one short measurement but through a series of measurements overmultiple pulses.

[0071] Pulse shape detector 1300 includes polarization controller 1310splitter (not shown), tunable delay device 1320, optical hybrid 1330,detectors 1340, analog-to-digital (A/D) converter 1350 and processor1360. The polarization controller 1310 is coupled to tunable delaydevice 1320 and optical hybrid 1330. Tunable delay device 1320 is alsocoupled to optical hybrid 1330. Optical hybrid 1330 is coupled todetectors 1340, which are coupled to A/D converter 1340. AID converter1350 is coupled to processor 1360, which is coupled to tunable delaydevice 1320. Note that polarization controller 1310 is an optionalcomponent and need not be present in other embodiments.

[0072] The incoming optical pulses pass through polarization controller1310 so that the optical pulses exiting the polarization controller 1310have only a single polarization such as for example, only verticalpolarization, horizontal polarization, clockwise circular polarizationor counterclockwise circular polarization. Splitter (not shown) splitsthe exiting optical pulses onto two different optical paths 1312 and1314. The optical pulses propagating on path 1314 are delayed by tunabledelay 1320 relative to the optical pulses on optical path 1312 by aperiod of time that is a fraction of the typical pulse width. Forexample, tunable delay 1320 can introduce a delay of about {fraction(1/10)} of the pulse width. The optical pulses delayed by tunable delaydevice 1320 are output on optical path 1324.

[0073] The optical pulses propagating on optical paths 1312 and 1324 arereceived by optical hybrid 1330. Optical hybrid 1330 can be, forexample, a 90° optical hybrid similar to those used for the signaldetection in coherent communication systems (see, for example, S. Betti,G. DeMarchis, E. Iannone “Coherent optical communications systems,” JohnWiley and Sons, Inc., 1995). Optical hybrid 1330 can be implemented, forexample, in fiber, silica, LiNbO₃ or other materials. Although opticalhybrid 1330 is shown in FIG. 13 as a 90° hybrid with four outputs, otherconfigurations are also possible.

[0074] The optical signals output from the optical hybrid 1330 aredetected by detectors 1340, which can be for example photodiodes.Although detectors 1340 are balanced photodetectors, in otherembodiments the detectors need not be balanced photodetectors. Detectors1340 produce electrical signals based on the received optical signalsand provide those electrical signals to A/D converter 1350. Although theembodiment in FIG. 13 shows four detectors 1340, other embodimentshaving a lesser or larger number of detectors are possible. The digitalsignals produced by A/D converter 1350 are provided to processor 1360,which can be for example a digital signal processor (DSP). For eachreceived optical pulse, processor 1360 can calculate a digital signalthat represents the extent to which the corresponding optical pulse onoptical path 1312 and the corresponding optical pulse 1324 overlap. Thisdigital signal can be stored in a buffer (not shown) that is accessibleby processor 1360. The measured optical pulse corresponds to aparticular delay applied by tunable delay device 1320.

[0075] The next measurement is based on a subsequently received opticalpulse where the portion of light on optical path 1314 has a differentdelay applied by tunable delay device 1320. For example, a delay on anadditional {fraction (1/10)} of a typical pulse width (e.g., {fraction(2/10)} for the second measured optical pulse) can be applied by tunabledelay device 1320. Now, the extent to which the optical pulse portion onoptical path 1312 and the optical pulse portion on optical path 1324overlap will differ based on this new delay value. Processor 1360produces another digital signal that represents this new overlap. Thisdigital signal is again stored in the buffer accessible by processor1360. This measurement procedure is repeated (i.e., an incrementallyincreased delay value is applied to subsequent optical pulses) until theoptical pulse on optical path 1312 and the optical pulse on optical path1324 substantially do not overlap. The effective pulse width is thencalculated from the digital signals stored in the buffer.

[0076] The pulse shape detector based on autocorrelation and describedin reference to FIG. 13 can find various applications in differentfields. For example, a pulse shape detector based on autocorrelation canbe used in optical communication systems regardless of the manner inwhich communication channels are multiplexed or the manner in which datais modulated. For example, a pulse shape detector based onautocorrelation can be used for optical signals in a wavelength-divisionmultiplexing (WDM) system, a time-division multiple access (TDMA) systemand a code-division multiple access (CDMA) system. In addition, a pulseshape detector based on autocorrelation can be used for optical signalshaving phase-modulated data, frequency-modulated data oramplitude-modulated data.

[0077] Moreover, the disclosed pulse shape detector based onautocorrelation is not limited to the pulse shapes having only three ormore harmonics, but can be applied to variety of pulse shapes such asGaussian or super-Gaussian pulse shapes. Because known devices thatmeasure the autocorrelation function (i.e., the pulse width) ofarbitrary short pulses are based on second harmonics generationcrystals, the disclosed pulse shape detector based on autocorrelationcan have a sensitivity on the order, more or less, of the known devicesbased on second harmonics generation crystals.

[0078]FIG. 14 shows a pulse shape detector, according to anotherembodiment of the invention. As shown in FIG. 14, pulse shape detector1400 includes a splitter device 1410, a dispersion device 1420 and abalanced detector 1430. Splitter device 1410 is coupled to dispersiondevice 1420 by optical paths 1412 and 1414. Dispersion device 1420 iscoupled to balanced detector 1430 by optical paths 1422 and 1424. Inalternative embodiments where the split optical path lengths differ(e.g., the length of optical paths 1412 and 1422 on the one hand differfrom the length of optical paths 1414 and 1424 on the other hand), atunable delay device can be disposed within one of the optical paths.

[0079] Splitter device 1410 receives an optical signal and divides theoptical signal onto two separate optical paths 1412 and 1414. Thedispersion device 1420 introduces a dispersion having one sign ontooptical path 1422 and the same amount of dispersion but with theopposite sign onto optical path 1424. In this embodiment, the dispersiondevice 1420 can be based on, for example, a chirped fiber Bragg grating.FIG. 15 shows a pulse shape detector with single chirped fiber Bragggrating, according to an embodiment of the invention.

[0080] As shown in FIG. 15, pulse shape detector 1500 includes tapcoupler 1550, splitter device 1510, dispersion device 1520, tunabledelay device 1530, variable optical attenuator (VOA) 1560 and balanceddetector 1540. Note that tunable delay device 1530 and VOA 1560 areoptional. Splitter device 1510 includes a 3-db coupler 1516. Dispersiondevice 1520 includes circulator 1526, chirped fiber Bragg grating 1528and circulator 1529. Tunable delay device 1530 includes a variableoptical delay (VOD) 1536. Although not explicitly shown, a low passfilter can be added to the pulse shape detector 1500 to suppressunwanted noise.

[0081] A small portion of the distorted optical pulse (i.e., thereceived optical signal after being transmitted through thecommunication link) is separated for the pulse shape measurement. Thissmall optical portion of the distorted pulse is separated, for example,by tap coupler 1550, which can be for example a 10:90 coupler. Theremaining portion of the distorted optical pulse can be provided to thereceiver for detecting the modulated data. The 3-dB coupler 1516 ofsplitter device 1510 splits the light onto two optical paths 1512 and1514.

[0082] Dispersion device 1520 then introduces a certain amount ofdispersion into the optical signals on each of these optical paths. Morespecifically, chirped fiber Bragg grating 1528 with circulator 1526introduces a certain amount of dispersion onto the optical pulsespropagating on optical path 1514. Similarly, chirped fiber Bragg gratingwith circulator 1529 introduces an equal amount of dispersion, but withan opposite sign, onto the optical pulses propagating on optical path1512. In this embodiment, the optical signals from both optical paths1514 and 1512 are reflected by the same chirped fiber Bragg grating1528. Because these optical signals are reflected from the oppositesides of the grating 1528, the sign of the introduced chirp is oppositefor the pulses from optical path 1514 and the pulses from optical path1512. The amount of chirp introduced is the same for both optical paths1514 and 1512.

[0083] In an alternative embodiment, the same chirp can be introduced bytwo independent chirped fiber Bragg gratings each being a piece of fiberwith the required dispersion characteristics. In this alternativeembodiment, one chirped fiber Bragg grating can be coupled to oneoptical path and the other chirped fiber Bragg grating can be coupled tothe other optical path.

[0084] Returning to FIG. 15, dispersion device 1520 provides the opticalsignals onto optical paths 1524 and 1522. VOA 1536 is coupled to opticalpath 1522 and is configured to vary the optical power of optical signalspropagating on optical path 1522. By varying the optical power ofoptical signals propagating on optical path 1522, the optical power ofthe optical signals can be balanced before being received at balanceddetector 1540. VOD 1536 is coupled to optical path 1524 and provides avariable optical delay for synchronizing the optical signals received atthe detector so that an incremental delay is applied as described inFIG. 14. The bandwidth of the balanced detector can be, for example,close to the symbol rate of the data represented by the received opticalsignals.

[0085]FIG. 16 shows a pulse shape detector with single chirped fiberBragg grating, according to another embodiment of the invention. Asshown in FIG. 16, pulse shape detector 1600 includes tap coupler 1650,splitter device 1610, dispersion device 1620, tunable delay device 1630,VOA 1660 and balanced detector 1640. Note that tunable delay device 1630and VOA 1660 are optional. Splitter device 1610 includes a 3-db coupler1616. Dispersion device 1620 includes circulator 1626, chirped fiberBragg grating 1628 and circulator 1629. Tunable delay device 1630includes a variable optical delay (VOD) 1636. Balanced detector 1640includes photodetectors 1641 and 1644, bandpass filters 1642 and 1645,Schottky diodes 1643 and 1646, and comparator 1647. Although notexplicitly shown, a low pass filter can be added to the pulse shapedetector 1600 to suppress unwanted noise.

[0086] Pulse shape detector 1600 is similar to pulse shape detector 1500shown in FIG. 15 except that balanced detector 1640 includes twophotodetectors, one on each of the optical paths. More specifically,photodetector 1641 is coupled to VOD 1636 and coupled in series tobandpass filter 1642 and Schottky diode 1643. Similarly, photodetector1644 is coupled to VOA 1660 and coupled in series to bandpass filter1645 and Schottky diode 1646. Both Schottky diodes 1643 and 1646 arecoupled to comparator 1647.

[0087] The pulse shape detectors shown in FIGS. 14 through 16 canprovide an error signal that indicates the amount of correction that thetransmitter can subsequently apply to send an optical pulse having asubstantially optimal pulse shape. In other words, the signal output bythe balanced detector can be used as an error signal at the receiverside of the communication link. If the pulse generator at thetransmitter side forms the pre-chirped pulse in such a manner that thepulse has spectrally symmetric shape after transmission through fiber(i.e., communication link), both signals input into the balanceddetector will have the same value. In such a case, correction at is notneeded. However, if the signals input into the balanced detector havedifferent values, a corresponding correction is can be provided via theerror signal to the transmitter (e.g., to computer 130, which controlsadaptive phase and attenuation control 120 and pulse generator 110 asshown in FIG. 1) so that a subsequently sent optical pulse has asubstantially optimal pulse shape.

[0088] In one embodiment, the computer at the transmitter (e.g.,computer 130 shown in FIG. 1) can analyzes the output signal from thepulse shape detector at the receiver, and adaptively tune the parametersof optical pulse generator at the transmitter side to correct the pulseshape. This correction of the pulse shape can overcome distortions tothe transmitted pulse caused by chromatic dispersion, nonlinear effectsand inter-channel interference. The result of the work of such softwareis shown in FIG. 17. FIG. 17a shows the received original pulse trainwith distorted pulse shapes (i.e., without any correction). FIG. 15bshows the pulse train after several iterations for pulse reconstruction.FIG. 15c shows the pulse train after even more iterations. FIG. 15dshows the reconstructed pulse train, which substantially coincides inshape with initial one. The pulse shapes were substantially completelyreconstructed as a result of the application of correction algorithm.

[0089]FIG. 18 illustrates a flowchart of the method for adaptive pulseshaping, according to an embodiment of the invention. This method can beimplemented, for example, by computer 130 and adaptive phase andattenuation control 120, shown in FIG. 1. The method is based on themaximizing of the power of the optical signal portion having thespectral component with frequency F. Maximal magnitude of the signal atfrequency F means that both (f+F) and (f−F) spectral components are inphase with the signal at main frequency f. For condition to besatisfied, the chirp introduced by the modulator at the pulse generatorat the transmitter is just correct to compensate the dispersion of thefiber, and no phase shift exists between the signal portion having thespectral component f and the signal portion having the spectralcomponents (f+F) and (f−F) at the receiver point.

[0090] This dithering method can be implemented, for example, on amicroprocessor or personal computer (e.g., computer 130 shown in FIG. 1)to control the pulse generation by maximizing the voltage output of thepulse shape detector (e.g., pulse shape detector 20). The analog voltageoutput of the power detector can be sampled and quantified to digitalsignal by an A/D converter (not shown). The computer (or microprocessor,e.g., computer 130) can then maximize this feedback signal by thedithering method depicted in FIG. 18. The D/A converter (not shown)converts the digital output signal from this method to an analog signal,which controls the phase modulator (e.g., within pulse generator 110).

[0091] As shown in FIG. 18, at step 1800, a center voltage, V₀, and adither voltage, ΔV, are initialized. At step 1805, a voltage, V₁′, isset to the center voltage, V₀, minus the dither voltage, ΔV. At step1810, the testing signal with V₁′ is sent to the phase modulator of thepulse generator (e.g., modulator 732 of pulse generator 700 shown inFIG. 7). At step 1815, the optical pulse based on the testing signal isreceived (for example, at pulse shape detector 20 as shown in FIG. 1).At step 1820, the voltage V_(in) of the modulation signal is detected.At step 1825, the obtained value V_(in) is saved in V₁ buffer.

[0092] At step 1830, a voltage, V₂′, is set to the center voltage, V₀.At step 1835, the testing signal with V₂′ is sent to the phase modulatorof the pulse generator (e.g., modulator 732 of pulse generator 700 shownin FIG. 7). At step 1840, the optical pulse based on the testing signalis received (for example, at pulse shape detector 20 as shown in FIG.1). At step 1845, the voltage V_(in) of the modulation signal isdetected. At step 1850, the obtained value V_(in) is saved in V₂ buffer.

[0093] At step 1855, a voltage, V₃′, is set to the center voltage, V₀,plus the dither voltage, ΔV. At step 1860, the testing signal with V₃′is sent to the phase modulator of the pulse generator (e.g., modulator732 of pulse generator 700 shown in FIG. 7). At step 1865, the opticalpulse based on the testing signal is received (for example, at pulseshape detector 20 as shown in FIG. 1). At step 1870, the voltage V_(in)of the modulation signal is detected. At step 1875, the obtained valueV_(in) is saved in V₃ buffer.

[0094] At step 1880, a determination is made as to which voltage V₁, V₂or V₃ is the maximum. If V₁ is the maximum of V₁, V₂ and V₃, then thecenter voltage, V₀, will be decreased by the dither voltage, ΔV. If V₂is the maximum of V₁, V₂ and V₃, then the center voltage, V₀ remainswith the same value. If V₃ is the maximum of V₁, V₂ and V₃, then thecenter voltage, V₀, will be increased by the dither voltage, ΔV.

[0095]FIG. 19 shows the test setup for measuring pulse propagation in alink with significant dispersion using pulse shape correction, accordingto an embodiment of the invention. More specifically, the test setup1900 includes a distributed feedback (DFB) laser 1905, pulse generator1910, signal generator 1915, temperature controller 1920, AMZI filter1925, phase modulator 1930, VOA 1935, coupler 1940, dispersive fiber1945, erbium-doped fiber amplifier (EDFA) 1950, filter 1955 andoscilloscope/OSA 1960. As shown in FIG. 19, the pulse generator is basedon the embodiment shown in FIG. 7. The test setup 1900 allows thecomparison of the propagation of pulse-shape-corrected pulses andnon-pulse-shape-corrected pulses.

[0096] DFB laser 1905 can provide an optical signal having an opticalcarrier (f_(o)) to pulse generator 1910. Pulse generator 1910 can bebased on, for example, the embodiment shown in FIG. 7. Pulse generator1910 can include, for example, a MZ modulator driven by a 6.25 GHzsinusoidal wave that generates two 6.25 GHz tones (±F) around 1546.9 nmof the optical carrier (f_(o)) received from DFB laser 1905. Theamplitude-modulated signal produced by pulse generator 1910 is then fedto an AMZI filter 1925, which can have, for example, a FSR of 12.5 GHz.The outputs of AMZI filter 1925 are sent to coupler 1940 (e.g., a 50/50coupler) and to phase modulator 1930, which is coupled in turn to VOA1935. AMZI 1925, phase modulator 1930, VOA 1935 and coupler 1940 havepolarization maintaining (PM) fibers.

[0097] The output of coupler 1940 represents the output of a transmitterand is directed to dispersive fiber 1945 (e.g., having about −850 ps/nmdispersion). The output from dispersion fiber 1945 is amplified by EDFA1950 and filtered by filter 1955 before being monitored by monitor 1960.Monitor 1960 can be for example an oscilloscope and an optical spectrumanalyzer (OSA).

[0098]FIGS. 20a and 20 b show an example of the output pulse waveform onan oscilloscope based on the test setup of FIG. 19. FIGS. 21a and 21 bshow an example of the output pulse spectrum on an OSA corresponding tothe output pulse waveform shown in FIGS. 20a and 20 b.

[0099] As shown in FIG. 20a, signal 2000 is the pulse output of thepulse generator shown at 6.25 GHz with a contrast ratio of 5.4 dB. Thecarrier-to-sideband ratio is about 15.7 dB as can be seen in FIG. 21a.Signal 2010 is the distorted pulse after propagating through dispersionfiber 1945. The contrast ratio of the distorted pulse is reduced to 2.9dB (down from 5.4 dB for the undistorted pulse (signal 2000) transmittedfrom the pulse generator 1910).

[0100]FIG. 20b shows the output pulse waveforms after being corrected.Signal 2020 is the output pulse waveform output from the pulse generator1910 based on a pulse shaping correction. Signal 2030 is the outputpulse waveform of an optical pulse having undergone a pulse-shapingcorrection and after propagating through dispersive fiber 1945. As FIG.20b shows, the pulse that underwent a pulse-shaping correction isgreatly improved over the pulse without pulse-shaping correction. Morespecifically, the pulse that underwent a pulse-shaping correction has amuch higher contrast ratio even after propagating through the dispersivefiber 1945. The carrier-to-sideband ratio of the pulse output from thetransmitter is 8.8 dB as shown in FIG. 21b. As these experimentalresults show, the waveform and spectrum for the shaped pulses aresubjected to much less distortions due to the link dispersion.

[0101] The inventors conducted another experiment relating to theoperation of the method for the adaptive pulse-shape correctiondescribed above in reference to FIG. 17. FIG. 22 shows a test setup fortesting the method for adaptive pulse-shape correction described inconnection with FIG. 17. Similar to the test setup shown in FIG. 19,test setup 2200 includes a distributed feedback (DFB) laser 2205, pulsegenerator 2210, signal generator 2215, temperature controller 2220, AMZIfilter 2225, phase modulator 2230, VOA 2235, coupler 2240, dispersivefiber 2245, EDFA 2250, photodetector 2255, filter 2260, RF detector2265, A/D converter 2270, computer 2275 and D/A converter 2280.

[0102] The test setup shown in FIG. 22 differs from the test setup shownin 19 in the signal detection portion and in the closed loop to correctthe pulse pre-chirp. After EDFA 2250, the received signal is detected byphotodetctor 2255. The tone 6.25 GHz (corresponding to the 6.25 GHzsignal added by pulse generator 2210) is obtained from the outputelectrical signal after using the filter 2260. The intensity of the toneis measured by RF detector 2265. This signal is converted into digitalby AID converter 2270, and digital signal processing is performed bycomputer 2275. The resulting corrected signal is converted back toanalog form by D/A converter 2280 and applied to phase modulator 2230 tocorrect the pulse shape. The dithering algorithm described in FIG. 18can be applied. As shown in FIG. 20b, the pulse-shape-corrected pulses(signal 2030) maintain their shape in time when adaptive shapecorrection is applied. In the absence of the adaptive shape correction,the pulse shape “breathes” in time in an undesired manner.

Conclusion

[0103] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. Thus, the breadth and scopeof the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

[0104] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. An apparatus for producing optical pulses for useon a transmission link, comprising: a light source configured to producean optical signal; and a pulse generator coupled to the light source,the pulse generator configured to receive, for a first channel, theoptical signal and a clock signal, the pulse generator configured tomodify the optical signal based on the clock_signal to produce anoptical pulse having a predetermined pulse shape, the clock signal beingassociated with the predetermined pulse shape, the predetermined pulseshape being based on a transmission characteristic of the transmissionlink.
 2. The apparatus of claim 1, wherein: the pulse generator includesa Mach-Zehnder modulator responsive to the clock signal, the clocksignal includes a swing voltage and a voltage bias, the swing voltageand the voltage bias being selected to substantially maximize a Q-factorassociated with the predetermined pulse shape and the transmission link.3. The apparatus of claim 1, wherein: the optical signal includes afirst spectral component; and the pulse generator includes a modulator,the modulator is configured to receive the optical signal and to producethe optical pulse having the first spectral component, a second spectralcomponent and a third spectral component, the second spectral componentand the third spectral component being based on the clock signal.
 4. Theapparatus of claim 3, wherein the pulse generator further includes: aphase shifter coupled to the modulator, the phase shifter configured toreceive a phase adjustment signal and the first spectral component ofthe optical pulse, the phase shifter configured to modulate the firstspectral component of the optical pulse based on the phase adjustmentsignal from the phase shifter.
 5. The apparatus of claim 4, wherein thepulse generator further includes: an asymmetric Mach-Zehnderinterferometer (AMZI) coupled to the modulator and the phase shifter,the AMZI receiving the first spectral component, the second spectralcomponent and the third spectral component of the optical pulse from themodulator, the AMZI configured to send the first spectral component ofthe optical pulse to the phase shifter; and a combiner coupled to thephase shifter and the AMZI, the combiner configured to combine the firstspectral component of the light pulse with a second spectral componentand a third spectral component of the optical pulse.
 6. The apparatus ofclaim 1, wherein the pulse generator further includes: a phase shiftercoupled to the modulator, the phase shifter configured to receive aphase adjustment signal, the second spectral component and the thirdspectral component of the optical pulse, the phase shifter configured tomodulate the second spectral component and the third spectral componentof the optical pulse based on the phase adjustment signal from the phaseshifter.
 7. The apparatus of claim 1, wherein: the light source is acomb generator configured to produce the optical signal having a firstspectral component, a second spectral component and a third spectralcomponent associated with the first channel; the pulse generatorincludes a demultiplexer, a modulator and a multiplexer, thedemultiplexer being coupled to the light source and the modulator, themodulator being coupled to the multiplexer; the demultiplexer isconfigured to send the first spectral component of the optical signal tothe modulator and to send the second spectral component and the thirdspectral component of the optical signal to the multiplexer; themodulator is configured to modify the first spectral component of theoptical signal; and the multiplexer is configured to combine themodified first spectral component of the optical signal, the secondspectral component and the third spectral component of the opticalsignal for the first channel to produce the optical pulse.
 8. Theapparatus of claim 7, wherein: the optical signal includes a pluralityof spectral components associated with a second channel.
 9. Theapparatus of claim 1, wherein: the pulse generator is configured toreceive a second optical signal and a second clock signal, the secondoptical signal is received subsequent to the optical signal, the secondclock signal is based on an error signal associated with the firstoptical pulse after being transmitted over the transmission link and thepredetermined pulse shape.
 10. The apparatus of claim 1, furthercomprising: a modulator coupled to the pulse generator, the modulatorconfigured to receive a data signal, the modulator configured to phasemodulate the optical signal or the optical pulse based on the datasignal.
 11. The apparatus of claim 10, wherein: the modulator isconfigured to modulate the optical signal or the optical pulse based onthe data signal according to phase-shift keying (PSK), and the firstchannel is a wavelength-division multiplexed (WDM) channel from aplurality of WDM channels, each WDM channel from the plurality of WDMchannels being associated with its own modulation signal.
 12. Theapparatus of claim 10, wherein: the modulator is configured to modulatethe optical signal or the optical pulse based on the data signalaccording to differential phase-shift keying (DPSK), and the firstchannel is a wavelength-division multiplexed (WDM) channel from aplurality of WDM channels, each WDM channel from the plurality of WDMchannels being associated with its own modulation signal.
 13. Theapparatus of claim 1, further comprising: a modulator coupled to thepulse generator, the modulator configured to receive a data signal, themodulator configured to amplitude modulate the optical signal or theoptical pulse based on the data signal.
 14. The apparatus of claim 13,wherein: the modulator is configured to modulate the optical signal orthe optical pulse based on the data signal according to on-off keying(OOK), and the first channel is a wavelength-division multiplexed (WDM)channel from a plurality of WDM channels, each WDM channel from theplurality of WDM channels being associated with its own modulationsignal.
 15. The apparatus of claim 1, wherein: the first channel is anoptical-time-division multiplexed (OTDM) channel from a plurality ofOTDM channels, each OTDM channel from the plurality of OTDM channelsbeing associated with its own optical pulse and its own modulationsignal.
 16. A method for generating optical pulses for opticalcommunications using a transmission link, comprising: receiving, for afirst channel, an optical signal and a clock signal, the clock signalbeing associated with a predetermined pulse shape, the predeterminedpulse shape being based on a transmission characteristic of thetransmission link; and modifying the optical signal based on the clocksignal to produce an optical pulse having the predetermined pulse shape.17. The method of claim 16, wherein: the clock signal including a swingvoltage and a voltage bias, the swing voltage and the voltage bias beingselected to substantially maximize a Q-factor associated with thepredetermine pulse shape and the transmission link.
 18. The method ofclaim 16, wherein: the optical signal is a continuous-wave (CW) signal;and the predetermined pulse shape being a quasi-return-to-zero(quasi-RZ) pulse shape.
 19. The method of claim 16, further comprising:forming the optical signal as a continuous wave and having the firstspectral component, the optical signal being modified so that theoptical pulse has the first spectral component, a second spectralcomponent and a third component, the second spectral component and thethird spectral component of the optical pulse being associated with theclock signal.
 20. The method of claim 16, wherein the modifyingincludes: splitting the optical signal onto a first optical path and asecond optical path, a first spectral component of the optical signalbeing associated with the first optical path, a second spectralcomponent and a third spectral component of the optical signal beingassociated with the second optical path; modifying a phase of the firstspectral component of the optical signal on the first optical path basedon a phase adjustment signal; and combining the first spectral componentof the optical signal associated with the first optical path with thesecond spectral component and the third spectral component of theoptical signal associated with the second optical path.
 21. The methodof claim 16, wherein the modifying includes: splitting the opticalsignal onto a first optical path and a second optical path, a firstspectral component of the optical signal being associated with the firstoptical path, a second spectral component and a third spectral componentof the optical signal being associated with the second optical path;modifying a phase of the second spectral component and the thirdspectral component of the optical signal on the second optical pathbased on a phase adjustment signal; and combining the first spectralcomponent of the optical signal associated with the first optical pathwith the second spectral component and the third spectral component ofthe optical signal associated with the second optical path.
 22. Themethod of claim 16, further comprising: forming the optical signal as apulsed wave and having a first spectral component, a second spectralcomponent and a third spectral component, the modifying step including:modifying a phase of the first spectral component of the optical signalto produce an optical pulse having the predetermined pulse shape. 23.The method of claim 16, further comprising: receiving, for the firstchannel, a second optical signal and a second clock signal, the secondclock signal being based on an error signal, the error signal based on atransmission of the optical pulse over the transmission link; andmodifying the second optical signal based on the second clock signal toproduce a second optical pulse having a second predetermined pulseshape, the second predetermined pulse shape being associated with thetransmission characteristic of the transmission link a time subsequentto a the predetermined pulse shape.
 24. The method of claim 16, furthercomprising: phase modulating the optical signal or the optical pulsewith data using phase-shift keying (PSK), the first channel being awavelength-division multiplexed (WDM) channel from a plurality of WDMchannels, each WDM channel from the plurality of WDM channels beingassociated with its own modulation signal.
 25. The method of claim 16,further comprising: phase modulating the optical signal or the opticalpulse with data using differential phase-shift keying (DPSK), the firstchannel being a wavelength-division multiplexed (WDM) channel from aplurality of WDM channels, each WDM channel from the plurality of WDMchannels being associated with its own modulation signal.
 26. The methodof claim 16, further comprising: amplitude modulating the optical signalor the optical pulse with data using on-off keying (OOK), the firstchannel being a wavelength-division multiplexed (WDM) channel from aplurality of WDM channels, each WDM channel from the plurality of WDMchannels being associated with its own modulation signal.
 27. The methodof claim 16, further comprising: time-division multiplexing the opticalpulse or the optical signal for the first channel with an optical pulseor an optical signal for a second channel, the first channel and thesecond channel from a plurality of optical-time-division multiplexed(OTDM) channels.
 28. An apparatus, comprising: a photodiode configuredto receive an optical pulse having a first spectral component, a secondspectral component and a third spectral component, the second spectralcomponent and the third spectral component being based on a clockfrequency, the photodiode configured to send a first signal having anamplitude and a spectral component with the clock frequency; a filtercoupled to the photodiode, the filter having a spectral responseassociated with the clock frequency; and a detector coupled to thefilter, the detector configured to send an error signal based on theamplitude of the first signal.
 29. The apparatus of claim 28, whereinthe filter is a bandpass filter having a spectral response, the clockfrequency being within the spectral response of the bandpass filter. 30.The apparatus of claim 28, wherein: the photodiode is configured toreceive a plurality of optical pulses including the optical pulse and asecond optical pulse, the optical pulse is an un-modulated trainingpulse, the second optical pulse is phase modulated with data.
 31. Theapparatus of claim 28, wherein: the clock frequency is a microwavefrequency; the first spectral component of the optical pulse has anoptical frequency; the second spectral component of the optical pulsehas its own frequency corresponding to a sum of the optical frequencyand the microwave frequency; and the third spectral component of theoptical pulse has its own frequency corresponding to a difference of theoptical frequency and the microwave frequency.
 32. The apparatus ofclaim 28, wherein: the amplitude of the first signal is associated witha pulse width of an optical pulse received from a transmission link; andthe error signal being associated with a difference between the pulsewidth of the optical pulse and a predetermined pulse width, thepredetermined pulse width being based on a transmission characteristicof the transmission link.
 33. A method for measuring an optical pulsereceived from a transmission link, comprising: detecting a signal basedon an optical pulse having a first spectral component, a second spectralcomponent and a third spectral component, the second spectral componentand the third spectral component being based on a clock frequency;filtering the detected signal to produce a first signal associated withthe clock frequency; and detecting an amplitude of the first signal. 34.The method of claim 33, wherein: the filtering is performed over abandpass spectral response, the clock frequency being within thespectral response of the bandpass filter.
 35. The method of claim 33,further comprising: receiving a plurality of optical pulses includingthe optical pulse and a second optical pulse, the optical pulse is anun-modulated training pulse, the second optical pulse is phase modulatedwith data.
 36. The method of claim 33, wherein: the clock frequency is amicrowave frequency; the first spectral component of the optical pulsehas an optical frequency; the second spectral component of the opticalpulse has its own frequency corresponding to a sum of the opticalfrequency and the microwave frequency; and the third spectral componentof the optical pulse has its own frequency corresponding to a differenceof the optical frequency and the microwave frequency.
 37. The method ofclaim 33, wherein: the amplitude of the first signal is associated witha pulse width of an optical pulse received from the transmission link;and the error signal being associated with a difference between thepulse width of the optical pulse and a predetermined pulse width, thepredetermined pulse width being based on a transmission characteristicof the transmission link.
 38. An apparatus, comprising: a tunable delaydevice configured to receive a first amplitude portion of an opticalsignal; an optical hybrid coupled to the tunable delay device, theoptical hybrid configured to receive a second amplitude portion of theoptical signal; a detector coupled to the tunable delay device; and aprocessor coupled to the detector, the processor being configured tocalculate a pulse width of the optical signal based on anautocorrelation of the first amplitude portion of the optical signal andthe second amplitude portion of the optical signal component.
 39. Theapparatus of claim 38, wherein: the first amplitude portion of theoptical signal including its own plurality of optical pulses, the secondamplitude portion of the optical signal including its own plurality ofoptical pulses, the tunable delay device is configured to iterativelyapply an incremental delay from a range of delays to each pulse from theplurality of optical pulses for the first amplitude portion of theoptical signal, and the processor configured to calculate the pulsewidth by measuring overlaps between each corresponding pulse from theplurality of optical pulses for the first amplitude portion of theoptical signal and from the plurality of optical pulses for the secondamplitude portion of the optical signal over the range of delays. 40.The apparatus of claim 38, further comprising: a polarization controllerand a splitter device coupled to the tunable delay device and theoptical hybrid, the first amplitude portion of the optical signal andthe second amplitude portion of the optical signal being associated witha first polarization of the optical signal.
 41. The apparatus of claim40, wherein: the optical hybrid is a ninety-degree optical hybrid havinga first port coupled to the splitter device, a second port coupled tothe tunable delay device and a third port coupled to the detector(actually there are four output ports coupled to four detectors, may betwo).
 42. An apparatus, comprising: a dispersion device configured toreceive a first portion of an optical signal on a first optical path anda second portion of the optical signal on a second optical path, thedispersion device configured to introduce a first dispersion into thefirst portion of the optical signal and a second dispersion into thesecond optical signal, the first dispersion having its own amplitude andsign, the second dispersion having its own amplitude and sign, theamplitude of the first dispersion being substantially equal to theamplitude of the second dispersion, the sign of the first dispersionbeing opposite of the sign of the second dispersion; and a balanceddetector coupled to the dispersion device.
 43. The apparatus of claim42, wherein: the dispersion device is a chirped Bragg grating having afirst side and a second side, the first side of the chirped Bragggrating being disposed within the first optical path, the second side ofthe chirped Bragg grating being disposed within the second optical path.44. The apparatus of claim 42, wherein: the dispersion device includes afirst chirped Bragg grating and a second chirped Bragg grating, thefirst chirped Bragg grating being disposed within the first opticalpath, the second chirped Bragg grating being disposed within the secondoptical path.
 45. The apparatus of claim 42, wherein: the dispersiondevice includes a first dispersion-compensating fiber (DCF) and a secondDCF, the first DCF being disposed within the first optical path, thesecond DCF being disposed within the second optical path
 46. Theapparatus of claim 42, further comprising: a first photodetector coupledto the first side of the chirped Bragg grating; a second photodetectorcoupled to the second side of the chirped Bragg grating; a firstbandpass filter coupled to the first photodetector; a second bandpassfilter coupled to the second photodetector; a first Schottky diodecoupled to the first bandpass filter and the balanced detector; and asecond Schottky diode coupled to the second bandpass filter and thebalanced detector.
 47. A method for measuring an optical signal receivedfrom a transmission link, the optical signal including a plurality ofoptical pulses having an estimated pulse width, comprising: splittingthe optical signal into a first amplitude portion associated with afirst optical path and a second amplitude portion associated with asecond optical path; delaying each pulse from the plurality of pulsesassociated with the first amplitude portion of the optical signal withan increasing amount of delay corresponding to a fraction of theestimated pulse width of the optical pulse; combining the firstamplitude portion of the optical signal with the second amplitudeportion of the optical signal using an optical hybrid; detecting anoverlap between the first amplitude portion of the optical signal andthe second amplitude portion of the optical signal; and determining apulse width of a pulse from the plurality of optical pulses for theoptical signal based on the detected overlaps between the firstamplitude portion of the optical signal and the second amplitude portionof the optical signal for the range of delays.
 48. The method of claim47, further comprising: storing the detected overlaps between the firstamplitude portion of the optical signal and the second amplitude portionof the optical signal, the range of delays including a final delaycorresponding to substantially zero overlap between a pulse of the firstamplitude portion of the optical signal and a corresponding pulse of thesecond amplitude portion of the optical signal.
 49. A method formeasuring an optical signal received from a transmission link, theoptical signal including a plurality of optical pulses having anestimated pulse width, comprising: introducing a first dispersion into afirst portion of an optical signal on a first optical path; introducinga second dispersion into a second portion of the optical signal on asecond path, the first dispersion having its own amplitude and sign, thesecond dispersion having its own amplitude and sign, the amplitude ofthe first dispersion being substantially equal to the amplitude of thesecond dispersion, the sign of the first dispersion being opposite ofthe sign of the second dispersion; and detecting the first portion ofthe optical signal after the introducing of the first dispersion and thesecond portion of the optical signal after the introducing of the seconddispersion to produce a balanced-detected signal.
 50. The method ofclaim 49, wherein: the first dispersion is introduced into the firstportion of the optical signal by a first side of a chirped Bragggrating; and the second dispersion is introduced into the second portionof the optical signal by a second side of the chirped Bragg grating. 51.The method of claim 49, wherein: the first dispersion is introduced intothe first portion of the optical signal by a first chirped Bragggrating; and the second dispersion is introduced into the second portionof the optical signal by a second chirped Bragg grating.
 52. The methodof claim 49, wherein: the first dispersion is introduced into the firstportion of the optical signal by a first dispersion-compensating fiber(DCF); and the second dispersion is introduced into the second portionof the optical signal by a second DCF.
 53. A method for adaptivelytuning a pulse generator sending an optical pulse over a transmissionlink, comprising: sending a plurality of testing signals each beingassociated with its own dithered value from a plurality of ditheredvalues, each dithered value from the plurality of dither values beingassociated with at least one from a center value and an offset value;receiving a plurality of optical pulses each being uniquely associatedwith an testing signal from the plurality of testing signals; detectinga plurality of modulation signals based on the plurality of opticalpulses, each modulation signal from the plurality of modulation signalshaving its own amplitude and a spectral component with a modulationfrequency; and calculating a new center value based on the amplitude ofmodulation signals.
 54. The method of claim 53, wherein: the pluralityof testing signals includes a first testing signal, a second testingsignal and a third testing signal, the sending includes: sending thefirst testing signal, the first testing signal being associated with adifference of a center value and an offset value; sending the secondtesting signal, the second testing signal being associated with thecenter value; sending the third testing signal, the third testing signalbeing associated with a sum of a center value and an offset value. 55.The method of claim 54, wherein the receiving includes: receiving anoptical pulse based on the first testing signal, the optical pulse forthe first testing signal being associated with its own amplitude;receiving an optical pulse based on the second testing signal, theoptical pulse for the second testing signal being associated with itsown amplitude; and receiving an optical pulse based on the third testingsignal, the optical pulse for the third testing signal being associatedwith its own amplitude.
 56. The method of claim 53, wherein: the newcenter value is calculated by selecting a maximum amplitude from theamplitude of modulation signal for the first testing signal, theamplitude of the modulation signal for the second testing signal and theamplitude of the modulation signal for the third testing signal.